1. Field of the Invention
The present invention relates to an image heating apparatus for use in an image forming apparatus such as a copying machine and a laser beam printer.
2. Description of the Related Art
An image heating apparatus for use in an image forming apparatus for heating and fixing uses a process of introducing a recording material as a material to be heated into a nip portion formed between a heating member maintained at a predetermined temperature and a pressure roller pressure-contacted with the heating member and heating the recording material while pinching and conveying the recording material. The image heating apparatus, particularly, the heating member of the image heating apparatus using a film heating process generally uses a heater with a resistance heat member formed on a substrate made of a ceramic material or the like.
When a heater with the same resistance value is used in the image heating apparatus located in an area of a 100-V commercial power supply or a 200-V commercial power supply, the maximum power suppliable to the heater in an area of the 200-V commercial power supply is four times that of the heater in an area of the 100-V commercial power supply. This is because the power supplied to the heater is proportional to the square of the voltage. The larger the maximum power suppliable to the heater, the worse the generation of a harmonic current, a flicker, and like by a heater power control such as a phase control and a wave-number control. In addition, considering a case in which the image heating apparatus causes thermal runaway, a more responsive safety circuit is required. For that reason, a heater with a different resistance value is often used in an image heating apparatus depending on the area of a 100-V commercial power supply or a 200-V commercial power supply. There has been proposed a method of switching the heater resistance value using a switch unit such as a relay as a method of implementing an image heating apparatus that can be shared in both areas of the 100-V commercial power supply and the 200-V commercial power supply. For example, Japanese Patent Application Laid-Open No. H07-199702 or U.S. Pat. No. 5,229,577 proposes an image heating apparatus having a first current path and a second current path extending in a longitudinal direction of the heater and a method of switching the heater resistance value by connecting the two current paths in series or in parallel. In order to switch the two current paths between a serial connection and a parallel connection, Japanese Patent Application Laid-Open No. H07-199702 describes a method of using a make contact (normally open contact) relay or a break contact (normally closed contact) relay and a BBM contact (break-before-make contact) relay. Note that instead of the BBM contact relay, two make contact relays or a make contact relay and a break contact relay may be used. U.S. Pat. No. 5,229,577 describes a method of switching using the two BBM contact relays.
Unfortunately, the image heating apparatus using the heater resistance value switching method described in Japanese Patent Application Laid-Open No. H07-199702 or U.S. Pat. No. 5,229,577 causes an increase in noise level of a noise terminal voltage due to power control (phase control) of the heater in a state in which the two current paths of the heater are connected in series.